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Demystifying Oxides of Nitrogen

by Steve Zack - OTC Technical Trainer

Oxides of Nitrogen (NOx) are one of the four pollutants emitted by internal combustion engines. The others are carbon Monoxide (CO), hydrocarbons (HC), and sulfur dioxides (SO2). Like the other three gasses, excessive amounts of NOx are the byproduct of engines operating under something other than ideal conditions. In theory, a perfect running engine would create only these non-toxic by-products - carbon dioxide (CO2), water vapor (H2O), and heat. But even under the best of circumstances, even the newest of engines don't operate under ideal conditions. This deteriorates progressively as engine components wear, and we still haven't developed an engine control computer that can instantly and fully adjust to changes in temperature and atmospheric pressure.

What is NOx?

NOx is an odorless colorless gas that is a natural byproduct of combustion. By understanding the conditions under which it is formed, you're in a much better position to use sophisticated tools, like your engine analyzer, to pinpoint the mechanical or electrical cause of the excessive NOx readings.

Nitrogen exists everywhere in its natural state. Approximately 77 to 78 percent of the air we breathe contains 2 atoms of nitrogen (N2). This is a stable gas that is inert, meaning that it does not easily change its composition or form with other gasses. That is, until it reaches temperatures of approximately 2300 degrees Fahrenheit, which is well within the range of a typical combustion chamber. At approximately 2300 degrees Nitrogen atoms separate, as a result they now have a natural tendency to oxidize or bond with atoms of oxygen. Another 21% of the air we (and our engines) breath is made up of 2 atoms of oxygen (O2). NOx is one atom of nitrogen and unknown (or "x") quantities of oxygen atoms.

The Making of NOx

As the intake valve opens, the low pressure area formed in the cylinder is quickly replaced by air at an atmospheric pressure at 14.7 psi. The air combines with hydrocarbons (HC) in gasoline to form the basis for the combustion used to drive the piston and generate power. During compression, as the piston begins to rise, the molecules of air and gasoline will rub together creating friction, which becomes heat. As the molecules absorb the heat, they expand. As the piston continues to rise, the pressure within the cylinder continues to increase, causing even greater friction, which causes the temperature to rise, allowing the gases to expand even further.

As they absorb the heat, the hydrocarbon molecules will separate into hydrogen and carbon; and oxygen molecules (O2) will separate into two individual oxygen atoms. With spark ignition, one oxygen atom will oxidize (combine) with two hydrogen atoms to create water, and two oxygen atoms will form with one carbon atom to create carbon dioxide. As oxidation increases, the newly created water and carbon dioxide create additional pressure, which the engine converts into usable power to turn the crankshaft and power the vehicle. As long as the combustion chamber temperature remains below 2300 degrees Fahrenheit, the nitrogen molecule will remain inert, which means it won't separate into individual nitrogen atoms and combine with other gases. But at 2300 degrees, bad things happen to basically good gasses. The nitrogen molecules separate, and bond with oxygen to form NOx.

Obviously the solution would be to keep cylinder temperatures below 2300 degrees Fahrenheit, and certainly considerable progress has been made in that regard. But the combustion area in the cylinder contains many "pockets" where temperatures will be significantly higher than in the rest of the cylinder. Any area not directly surrounded by a cooling jacket is an obvious candidate.

To help prevent temperatures conducive to the formation of NOx, engine manufacturers have installed an EGR (Exhaust Gas Recirculation) valve and passage to recycle exhaust gas to cool the combustion chamber down. Over the years they have also lowered compression ratios to reduce friction of the gases, thus lowering combustion temperature. A richer fuel mixture also creates additional hydrocarbons to absorb the heat, and retarded timing advance is another part of the strategy to reduce combustion chamber temperatures. The downstream catalytic converter has a rhodium cell used to convert NOx to nitrogen and carbon dioxide.

Using a 5-Gas Analyzer to Diagnose NOx Causes

NOx can be formed by many component failures and system malfunctions - anything that allows the combustion chamber temperature to exceed 2300 degrees Fahrenheit. For example, if the cooling system is utilizing pure water, it will absorb combustion heat and obtain its boiling point rapidly, leaving an air pocket around the combustion chamber. The air pocket will become a hot spot allowing for the combustion chamber temperature to rise. Equally bad is 100% antifreeze, which forms a blanket around the combustion chamber, keeping heat in and allowing the combustion chamber temperature to rise. Rust surrounding the cooling jacket surface will create the same blanket around the combustion chamber. Poor flow through the radiator, as a result of a blockage or poor circulation, or a partially closed thermostat or limited flow from the water pump, will prevent the high temperatures from escaping through the cooling system. This temperature increase will cause preignition, allowing the oxygen to prematurely oxidize the hydrogen, reducing the available air and fuel during spark ignition. As a result, oxygen will be limited to complete the oxidation of the hydrogen and formation of carbon and oxygen into carbon dioxide will be limited. In this case the gas meter will read high HC, low CO, low 02, low C02, and high NOx.

As the piston rises during the compression stroke, the opposite force of compression in the upward direction and the downward force created as a result of preignition will cause the piston to rock, slapping the piston skirt against the cylinder wall, causing a knock. As a result of an engine knock, we should see the scanner knock reading indicate yes, with a command to retard ignition timing. The secondary ignition scope should have a higher than normal firing line. With excessive preignition, the early flame front comes in contact with the spark ignition flame front. This will result in high HC, low CO, high 02, low C02, and high NOx. The horsepower of the engine will suffer as well.

A lean condition as a result of a false signal from the oxygen sensor, an out-of-calibration MAP sensor, plugged injector, low fuel pressure, low fuel pump volume, or a vacuum leak will cause NOx as well. Under these conditions the oxygen will oxidize the hydrogen and the carbon, creating extreme combustion chamber temperature. This acts much like a cutting torch used to oxidize the carbon atom of the metal. As the temperature climbs the nitrogen atoms will separate and form with the oxygen atoms to create NOx. As the oxygen is used to form NOx, the oxygen will be used up, causing a lot of hydrocarbons to be unused. The gas analyzer will read high HC, low C0, high 02, low C02, and high NOx. The secondary ignition scope should have a higher than normal firing line, and longer than normal spark duration. The scanner would read zero to low oxygen sensor voltage and a lean condition. As a result of running lean, the additional HC from a richer mixture is not there to absorb the heat. Therefore, the CO molecule needed in the catalytic converter to cause the catalyst to reduce NOx to nitrogen and carbon dioxide will not be created.

If the EGR system were not functioning properly, as a result of a plugged or partially plugged passage, inoperative EGR valve, inoperative vacuum control system, or an electronic malfunction, the controlled exhaust flow would not occur. This would allow the combustion temperature to rise. The gas analyzer would see high HC, low CO, low O2, low CO2, and high NOx as result of a faulty EGR.

Carbon build-up on top of the piston or on the cylinder walls may also cause preignition. This would give similar results to the cooling system issue. The carbon build-up could be as a result of running extremely rich for any length of time. Therefore, when repairing high CO emissions failures, always assume that carbon has formed. This type of carbon can even be caused by oil consumption, which will also cool the combustion chamber temperature. Carbon on the throat of the valve will absorb fuel, causing a lean condition and giving a similar result as to the previous lean condition. This type of diagnosis may require a borescope to visually inspect pistons, cylinder bores or valves. To repair this, a chemical top end cleaner may help. If the vehicle is not running rich, the gas analyzer will read high HC, low CO, high O2, low CO2, and high NOx. The reason for the high O2 is that the carbon will assimilate a lean ignition misfire. The scanner would read low oxygen sensor voltage. The secondary ignition scope should have a higher than normal firing line, and a possible appearance of second firing line in the spark line.

A worn or slipping timing belt can certainly increase internal temperatures. If the timing belt or timing chain has excessive slack, the cam timing will be retarded. The camshaft will be behind the crankshaft resulting in the camshaft lobes not opening the valves in the proper relationship to the piston. The intake valves during the intake stroke will open late, causing the air to continue entering the cylinder later than required. As a result, the compression pressure will increase at the top of the compression stroke and temperature will reach its maximum later on into the stroke causing a longer oxidation period which results in extreme combustion temperature. As a result of late intake valve opening, the vacuum will be low, causing the MAP sensor to see a load adding more fuel, causing high CO. The gas analyzer will read high HC, high CO, low O2, low CO2, and high NOx.

If the base timing is advanced too far, the spark plug will ignite the air fuel mixture early, causing the combustion temperature to rise as the compression stroke continues, causing NOx to form. With a very early flame front in the compression stroke, as a result of advanced ignition timing, the temperature created as a result of early oxidation will cause the temperature created during compression to rise extensively.

The exhaust valve seating surface is not just to seal the cylinder airtight, but to provide a means of removing heat from the valve and disbursing it to the cooling system. An exhaust valve that is improperly seated will not transfer heat. As a result, the valve and its seat will rise in temperature, causing preignition. This will result in high HC, low CO, high O2, low CO2, and high NOx.

So as you can see, excessive NOx is caused by high internal temperatures. Unfortunately, there are any number of factors that can increase combustion chamber temperatures past the magic 2300 degrees. Fortunately, with your trusty five gas analyzer and a little reasoning ability, you're in great shape to isolate the problem, replace the faulty components or make adjustments as needed, and send the motorist on their way with a better performing and lower polluting vehicle.

copyright SPX 2011